I bought the 100GB Toshiba drive with 16MB of cache. It's a 5400RPM drive. I was hoping to find a Hitachi 7K100 somewhere, but *everyone* is out of stock.
The 7K100 is Hitachi's second-generation line of 7200RPM notebook drives. The 100GB model looks sweet, and is supposed to be up to 30% faster than the 7K60 60GB drive.
But even with my 5400RPM drive, I don't notice any lag issues or anything with Far Cry or HL2. It seems about as peppy as my desktop system, which has a 7200RPM drive in it.
All I can say is... i9300/XPS2 kicks ass!